Not much is known about Kate Warne prior to the day she walked into the Pinkerton National Detective Agency in 1856.
Answering an ad in a local newspaper, Warne went to Pinkerton's Chicago office and asked to see Allan Pinkerton about a job. There is still debate whether or not she intended to become a detective or a secretary. There were no women detectives until well after the Civil War. Pinkerton himself claimed that she demanded to become a detective.
According to Pinkerton's records, he was surprised to learn Kate was not looking for clerical work, but was actually answering an advertisement for detectives he had placed in a Chicago newspaper. Pinkerton said, 'It is not the custom to employ women detectives.' Kate argued eloquently that women could be 'most useful in worming out secrets in many places which would be impossible for a male detective.' A woman would be able to befriend the wives and girlfriends of suspected criminals and gain their confidence.
Her argument impressed Pinkerton and on August 23, 1856, he employed Kate Warne, over the strong objections of his brother Robert who was also his partner in the business. According to Pinkerton family history, Allan was smitten with the woman; she became his mistress and traveled with him. (He had a wife and children at home.) This caused problems when his brother questioned some of the expenses she turned in to the agency.
Thus, Warne became the first female private detective in the United States. Moreover, Pinkerton soon hired other females. Their ranks grew, Kate having shown Pinkerton their intrinsic value to his organization, and he appointed Warne Supervisor of Female Detectives.
Warne became a very good private investigator and acted undercover, infiltrating social events and gathering information no man could have obtained. She wore disguises, changed her accent at will and became a huge asset to the Agency.